We have continued studies on oxidative modification of proteins and enzyme inactivation during protein turnover and aging in an effort to understand the possible physiological role of this process. Oxidative inactivation of enzymes mediated by mixed-function oxidation systems is accompanied by the formation of protein carbonyl derivatives. We have used this property and developed several assays to detect and quantitate the levels of oxidized protein in tissue extract preparations from young and old animals. It is likely that these studies will permit us to identify and isolate oxidatively modified proteins from biological systems.